MOD 9 - Exposure and IQ in Digital Imaging Systems

Learning Objectives

  • Contrast the terms "exposure" and "dose"

  • Relate dose to image quality

  • Recognize factors that affect image quality.

  • Describe the factors that affect image quality.

  • Describe methods for changing image quality.

  • Explain how the imaging process affects image quality.

  • Explain how the technologist can control image quality.

Image Quality

subjective aspects:

  • viewer's preferences

  • visual acuity

Contrast Resolution

= the ability of the imaging system to express slight attenuation differences between similar adjacent tissues

= difference between the brightness of two adjacent structures

  • therefore images containing bone, fat, and air as the primary components will display a higher range of attenuation and thus a higher visualized contrast

  • oppositely with muscle, water and fat these items attenuate to a similar degree and thus will display with lower contrast

Greyscale

= the range of different brightness levels within an image

High contrast resolution

= images will have sharp differences between structures but not be able to differentiate similar attenuating tissues

  • smaller grey scale

Low contrast resolution

= images will be better at defining these similarly attenuating tissues

  • extensive grey shades

  • Dynamic Range (long grey scales)

= range of exposures that can be captured by a detector

= detector ability to accurately capture the range of photon intensities that exit the patient

  • DR have wide range of radiation intensities

  • this explains how little over/under exposures won’t show substantial decrease in IQ

Spatial Resolution

= the measure of a system's ability to accurately demonstrate small objects as distinct, often considered high detail or high resolution

  • All radiographic images have some degree of unsharpness

  • DR systems have fixed spatial resolution determined by the detector element size

  • In CR imaging the spatial resolution is determined by the frequency of sampling of each area of the plate being laser stimulated

Spatial resolution and DR

  • DR is inferior when compared to the spatial resolution in film screens

DR benefits

  • improved detector design

  • improved spatial frequency

  • superior contrast resolution

  • patient dose savings

  • elimination of film processing and storage more than compensate for this deficiency

How is a systems spatial resolution measured?

  • Line Pair Testing and Spatial frequency (determined from pixel pitch)

  • Modulation Transfer Function (MTF)

Spatial Frequency

= tests the system's ability to distinguish adjacent small objects as separate

  • usually assessed by the 'line pair' test tool that contains pairs of thin dense objects (wire) with increasing narrow spaces between the lines

  • spatial frequency = line pair = defined as both the solid line and the adjacent space

  • higher spatial frequency = higher spatial resolution

  • higher spatial frequency of the anatomy = smaller in size = reduced modulation and more difficult to image

Modulation Transfer Function (MTF)

= the ability of a system to accurately demonstrate small objects accurately

  • most common method of describing spatial resolution

  • ideal MTF = 1 (meaning the system expresses very small objects exactly as they exist, 0 = the object is not represented at all)

    • lower MTF = blurrier images

      • High spatial frequency objects/high spatial resolution/smaller objects → harder to image and thus look blurrier

  • MTF ranges 0-1

  • MTF can never be 1 in diagnostic imaging due to a variety of geometric factors and detector element size limitations (impossible)

Image Noise

= a blanket title that defines all destructive data being included in an image

Electronic Noise (dark current noise)

= inherent noise in digital imaging systems due to the use of electrical devices

  • managed by manufacturers through the use of fiber optics and direct transmission methods

  • display systems, CCD’s, II

Quantum Mottle

= another form of image noise, is directly related to dose and receptor exposure

  • Depending on patient factors, this may have both a direct and an inverse relationship with patient dose

  • Visible as brightness fluctuations and is photon-dependent

    • incident beam lacks sufficient energy = high absorption (photoelectric effect) → insufficient beam transmission = increased noise and patient dose

    • low-signal-to-noise ratio = noisy images that are produced when the detector pixels do not receive sufficient signal to produce an accurate sample

      • higher signal to noise ratio = more signal (useful diagnostic information), less noise

Exposure Latitude

= a digital image processing algorithm which is associated with a histogram of ideal exposure intensities that during processing can correct a wide variation in actual exposure

  • BUT DR systems cannot compensate for excessive noise caused by quantum mottle

Detector Saturation

= when the detector elements are flooded with transmitted photons (high exposure) and the differential attenuation required for radiographic is lost

Detective Quantum Efficiency (DQE)

= represents the detector absorption efficiency for a wide range of photon energies

  • detector signal-to-noise ratio is influenced by the DQE

  • DQE depends on the detector materials and design

  • DQE of 1 = a 100% probability of photons being detected

  • in all DR systems, DQE drops with increasing kV

  • high DQE = fewer x-ray photons are required to produce an image

  • Direct DR that uses amorphous selenium (a-Se) has the highest DQE

    DQE

Dynamic Range and Exposure Latitude

Dynamic Range

= The DR system's ability to express its contrast resolution

= the number of gray shades that a system is capable of representing on an image

= range of exposures that can be captured by a detector

= detector ability to accurately capture the range of photon intensities that exit the patient

  • DR have wide range of radiation intensities

  • A deep contrast resolution range is a strength of DR imaging

  • DR systems have a dynamic range of over 16,000 gray shades which requires highly sensitive detectors with high bit depth

  • DR has a wide exposure latitude = wide range of exposures that make an acceptable image

    • allows moderately overexposed or underexposed images to be processed to display acceptable diagnostic quality

Basic Windowing

Windowing

= allows the viewer to alter the grayscale display to improve visualization of different densities

  • a post-processing function

  • done by altering the window width (WW- number of gray shades represented) or the window level (WL- range of gray shades on the scale represented)

  • Image contrast is influenced heavily by quantum mottle

Window Level

= located on the display monitor which allows the image brightness to be increased or decreased throughout the entire range of densities

Things to Note when Windowing

  • done at the workstation monitor through the mouse function

  • important to return the image to the original grayscale before sending to PACS to ensure any adjustments are not saved

Image Brightness Adjustments

→ Adjusting the window level means that the center of the range of gray shades is moved up or down the scale → this will either allow more light gray shades or more dark gray shades to be expressed on the screen = decreased/increases brightness

Contrast Adjustments/Window Width

  • Increasing the number of gray shades represented on an image (widening the window width) = decreases image contrast

  • reducing the window width, image contrast can be improved

Image Brightness and Digital Imaging

  • Image brightness is controlled by our software processing system and also can be adjusted at the monitor and image viewer level

  • image brightness is determined through the attenuation of x-ray in the tissue

  • adequate brightness = proper attenuation of xray beam

    • bright = white = lots of attenuation

    • dark = black = no attenuation